The efficient use of recirculated exhaust gas (“EGR”) is important to modern internal combustion engines, including both gasoline and diesel fueled engines. Efficient use of EGR generally supports the objectives of realizing high power output from these engines while also achieving high fuel efficiency and economy and while meeting increasingly stringent engine-out emission requirements. The use of forced induction, particularly including exhaust gas driven turbochargers, is also frequently employed to increase the engine intake mass airflow and the power output of the engine by using waste energy derived from the exhaust gas. The efficient use of EGR and turbocharged forced-induction necessitates synergistic design of these systems.
The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”), and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine's exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components. An exhaust treatment technology in use for high levels of particulate matter reduction is the diesel particulate filter device (“DPF”). There are several known filter structures used in DPF's that have displayed effectiveness in removing particulate matter from exhaust gas such as ceramic honeycomb wall-flow filters, wound or packed fiber filters, open cell foams, sintered metals, fibers, etc.
The filter structure, regardless of the type used, is a physical structure for removing particulates from the exhaust gas and, as a result, the accumulation of filtered particulates will have the effect of gradually increasing the exhaust system backpressure experienced by the engine. To address backpressure increases caused by the accumulation of exhaust gas particulates, the DPF is periodically cleaned or regenerated. Regeneration of a DPF, particularly in vehicular applications, is typically automatic and is controlled by an engine or other controller based on signals generated by engine and exhaust system sensors. The regeneration event involves increasing the temperature of the DPF filter structure to levels that are often between 500° C. and 650° C. in order to burn the accumulated particulates. One method of generating the temperatures required in the exhaust system for regeneration of the DPF is to deliver unburned HC to an oxidation catalyst device that is disposed upstream of the DPF. The HC may be delivered to the exhaust system by adjusting the fuel injection, timing or both of the engine and “over-fueling” the engine resulting in unburned HC exiting the engine with the exhaust gas and entering the exhaust system. In the alternative, a fuel injector associated with a fuel system may be fluidly connected to the exhaust system upstream of the oxidation catalyst device for delivery of HC directly to the exhaust gas. The HC is oxidized in the oxidation catalyst device resulting in an exothermic reaction that raises the temperature of the exhaust gas to a level sufficient to burn the accumulated particulates in the DPF.
A disadvantage to this method of regeneration is that the inlet for the EGR system is typically upstream of the oxidation catalyst and DPF so that the EGR contains both unburned HC and particulate matter. The delivery of unburned HC to an EGR system may lead to clogging of the system, especially if the EGR system employs an exhaust gas cooler. Additionally, recirculation of HC's through the EGR system and back into the engine intake system reduces the quantity of HC available for DPF regeneration and reduces the fuel efficiency of the engine. One method for avoiding the introduction of unburned HC to the EGR system is to limit engine over-fueling to specific engine cylinders with un-fueled exhaust directed to the EGR system and HC-laden exhaust directed to the DPF during regeneration. In a V-configured engine, the over-fueling may involve one bank of cylinders and in an in-line engine it may involve a group of cylinders (cylinders 1, 2, 3 in an in-line 6 cylinder engine for example). A disadvantage to the above method of over-fueling the engine is that the exhaust flows are permanently separated, resulting in excess un-fueled exhaust bypassing the turbocharger when not required by the EGR system. For improved efficiency, it is desirable to take advantage of the full exhaust flow volume, less that diverted for EGR purposes, to power the turbocharger in an engine using forced induction.